The present invention relates generally to carrier phase measurements, and more particularly, relates to lock s lip detection.
A Global Positioning System (GPS) is a satellite-based system that provides position, velocity, and time infonrmation with a high degree of accuracy. The GPS consists of a number of orbiting satellites and receivers. Currently there are twenty-four satellites revolving in six orbital planes. The orbiting satellites broadcast a continuous series of radio signals, which are detected by the receivers.
The radio signals contain information regarding the known position of the satellites, the distance to each satellite, and the relative velocity of the satellites with respect to the receiver. With information from four or more satellites, the receiver location can be determined. For more accurate position measurements, two or more receivers may be used together to provide differential correction. A system using two or more receivers is referred to as Differential GPS (DGPS).
A high-performance GPS receiver can provide carrier phase measurements. The carrier phase measurements are formed by a phase lock loop (PLL) and are derived from the comparison of the incoming satellite broadcast with a phase of an oscillator located within the receiver at an epoch of measurement. While the accumulated difference between the received transmissions and the receiver phase can be measured and monitored, it is not possible to measure the number of waves between the receiver and the satellite at an instance of first observation. This unknown quantity is called integer ambiguity and requires ambiguity resolution in order to use the carrier phase measurements as range measurements. Ambiguity resolution may be performed using a variety of methods including integer searches and motion-based approaches. Furthermore, ambiguity resolution may use as aids multiple antenna pairs, multiple GPS observables, and external aiding.
While ambiguity resolution can be thought of as enabling the use of carrier phase information as an absolute range measurement between a GPS receiver""s antenna and a satellite, in a practical sense ambiguity resolution is typically used to determine relative range between two or more GPS antennas, and not to determine the absolute range between a receiver and a satellite. This document will continue to refer to the range determined via ambiguity resolution generically as a range.
Once ambiguity resolution has been accomplished, the GPS carrier phase measurements can be used as very accurate range measurements. The ambiguity resolution will remain valid for each satellite as long as the GPS receiver maintains a lock on the GPS carrier signal from the satellite. However, if there is a loss of carrier phase lock or xe2x80x9clock slip,xe2x80x9d ambiguity resolution must be executed again. Lock slip may be caused by obstructions to the satellite signal, low signal to noise ratio, incorrect processing within the receiver software, high antenna acceleration, interference from other radio signal sources, or high ionospheric activity.
The use of carrier phase measurements in commercial aviation is becoming ore applicable due to the adoption of microelectromechanical system (MEMS) inertial sensors in Inertial Navigation Systems (INS). While MEMS inertial systems can adequately provide pitch and roll solutions, they are incapable of providing a heading solution. Using GPS carrier phase measurements to compute an attitude solution, including but not limited to heading, may solve this shortcoming. Commonly referred to as GPS attitude determination, this approach utilizes the carrier phase measurements from a number of pairs of GPS receivers/antennas rigidly mounted on a vehicle body to compute an attitude solution based on the precise relative position of the GPS antenna pairs. The multiple antennas may all be connected to a single GPS receiver or there may be an individual GPS receiver for each antenna. This precise relative solution is possible due to the availability of carrier phase range measurements, which are enabled by ambiguity resolution.
It is important that the GPS attitude determination algorithm be aware of any lock slips. If a lock slip occurs and ambiguity resolution is not executed again, the GPS attitude solution may be incorrect. While most receivers provide a xe2x80x9clock slip indicationxe2x80x9d along with their carrier phase measurements, a more reliable method is needed.
For applications that integrate INS with GPS, the inertial information from the INS can provide a more reliable and self-contained method for lock slip detection.